Gas Optimizations

[code423n4]

-- 1
All functions that receive uints which are not uint256 could be changed to uint256 and only casted to the intended size when storing it.
More info at: ourzora/v3#125 (comment)

Just swapped the lend() function and consequent related tests resulting in: (...) Overall gas change: -11839 (-0.041%)
But there are many other functions (eg. createLoan) which could use such change, so the gas saving should be way more than that.

-- 2
cache borrowTicketContract at createLoan to save one SLOAD on a successful transaction.

-- 3
use cached previousInterestRate on calling _interestOwed

Overall gas change: -60 (-0.000%)

diff --git a/contracts/NFTLoanFacilitator.sol b/contracts/NFTLoanFacilitator.sol
index 46d6ef5..bb25958 100644
--- a/contracts/NFTLoanFacilitator.sol
+++ b/contracts/NFTLoanFacilitator.sol
@@ -165,6 +165,7 @@ contract NFTLoanFacilitator is Ownable, INFTLoanFacilitator {
             // will underflow if amount < previousAmount
             uint256 amountIncrease = amount - previousLoanAmount;
 
+            uint256 accumulatedInterest;
             {
                 uint256 previousInterestRate = loan.perAnumInterestRate;
                 uint256 previousDurationSeconds = loan.durationSeconds;
@@ -177,14 +178,14 @@ contract NFTLoanFacilitator is Ownable, INFTLoanFacilitator {
                 || (previousInterestRate != 0 // do not allow rate improvement if rate already 0
                     && previousInterestRate - (previousInterestRate * requiredImprovementRate / SCALAR) >= interestRate), 
                 "NFTLoanFacilitator: proposed terms must be better than existing terms");
-            }
 
-            uint256 accumulatedInterest = _interestOwed(
-                previousLoanAmount,
-                loan.lastAccumulatedTimestamp,
-                loan.perAnumInterestRate,
-                loan.accumulatedInterest
-            );
+                 = _interestOwed(
+                    previousLoanAmount,
+                    loan.lastAccumulatedTimestamp,
+                    previousInterestRate,
+                    loan.accumulatedInterest
+                );
+            }
             require(accumulatedInterest <= type(uint128).max,
             "NFTLoanFacilitator: accumulated interest exceeds uint128");

-- 4

cache loan.loanAmount on repayAndCloseLoan

testRepayInterestOwedExceedingUint128() (gas: -253 (-0.001%)) 
testRepayAndCloseSuccessful() (gas: -253 (-0.001%)) 
testRepayAndClose() (gas: -253 (-0.003%)) 
Overall gas change: -759 (-0.004%)

diff --git a/contracts/NFTLoanFacilitator.sol b/contracts/NFTLoanFacilitator.sol
index f1f570f..ff2e3f7 100644
--- a/contracts/NFTLoanFacilitator.sol
+++ b/contracts/NFTLoanFacilitator.sol
@@ -231,23 +231,24 @@ contract NFTLoanFacilitator is Ownable, INFTLoanFacilitator {
     /// See {INFTLoanFacilitator-repayAndCloseLoan}.
     function repayAndCloseLoan(uint256 loanId) external override notClosed(loanId) {
         Loan storage loan = loanInfo[loanId];
+        uint256 _loanAmount = loan.loanAmount;
 
         uint256 interest = _interestOwed(
-            loan.loanAmount,
+            _loanAmount,
             loan.lastAccumulatedTimestamp,
             loan.perAnumInterestRate,
             loan.accumulatedInterest
         );
         address lender = IERC721(lendTicketContract).ownerOf(loanId);
         loan.closed = true;
-        ERC20(loan.loanAssetContractAddress).safeTransferFrom(msg.sender, lender, interest + loan.loanAmount);
+        ERC20(loan.loanAssetContractAddress).safeTransferFrom(msg.sender, lender, interest + _loanAmount);
         IERC721(loan.collateralContractAddress).safeTransferFrom(
             address(this),
             IERC721(borrowTicketContract).ownerOf(loanId),
             loan.collateralTokenId
         );
 
-        emit Repay(loanId, msg.sender, lender, interest, loan.loanAmount);
+        emit Repay(loanId, msg.sender, lender, interest, _loanAmount);
         emit Close(loanId);
     }

[wilsoncusack]

1. we would need then to check whether the passed param does not exceed the max value for the struct value, which erases gas gains. Not ok with just covering to e.g. uint32.max if it exceeds
2. ack
3. ack

[wilsoncusack]

with-backed/backed-protocol#62